Mass spectrometry is an analytical technique in which molecules of a sample are ionized in a vacuum and separated according to their mass charge ratio (m/Q, wherein m is the mass in amu and Q is the charge in units of electron charge). The number of ions having the same mass charge ratio within the resolution capacity of the equipment are counted and are typically reported as a peak on a mass spectrum having a horizontal position which corresponds to the m/e of the ions and a height which corresponds to the quantity of ions.
When a molecule of sample is ionized, it tends to break apart and produce a collection of ions which is characteristic of the parent molecular structure. Mass spectrometers with sufficient resolution capable of resolving and counting each ion. The resulting spectrum is effectively a fingerprint of the sample. High resolution mass spectrometers are further capable of determining the composition of a sample by resolving the mass to charge ratio of the parent ion so precisely that it can be distinguished from all other possible parent species.
The most common type of mass spectrometer resolves ions of different m/e by accelerating them to the same kinetic energy and then passing them through a magnetic field. In these magnetic instruments, resolution varies directly with the size of the magnet. Even moderate resolution devices are cumbersome, delicate and expensive. Accordingly, moderate and high resolution magnetic instruments have been largely confined to laboratory applications.
Another method of resolving ions by mass per charge is known as time of flight mass spectrometry (TOFMS). In a TOF mass spectrometer, the ions are accelerated to the same kinetic energy, allowed to traverse a flight path through a defined region and picked up by a detector at the other end of the flight region. TOFMS takes advantage of the fact that ions of different masses and equal initial energy that have been accelerated to the same kinetic energy travel at different velocities, as expressed in the equation E.sub.K =QV=1/2Mv.sup.2, wherein V is the acceleration voltage, E.sub.K is the ion's kinetic energy; Q is its charge in units of e (1.6.times.10.sup.-19 coul.); M is its mass; and v is its velocity. TOF mass spectrometers resolve ions by the time it takes them to traverse the flight region. Accordingly, TOF mass spectrometers do not require a magnet or the precise magnetic field variation control circuitry of magnetic instruments. This makes size reduction of TOF instruments for field use potentially more feasible than size reduction of magnetic instruments. Unfortunately, the resolution of TOF instruments is critically dependent upon accurate and precise time of flight detection and the reduction in flight times resulting from a shortened flight path has seriously limited the resolution presently attainable with compact instruments.
For accurate time of flight measurement, an instrument having a 20 cm flight region must be provided with a means to initialize a time measurement within nanoseconds of the moment of sample ionization. Second, the sample must be highly planar and normal to the flight path. And third, the detector must have good time resolution capability, i.e. a sufficiently fast rise time to detect ion impacts. Electronics and detector must recover sufficiently fast to record subsequent impacts. The effects of nonideal timing and sample alignment can be mitigated by lengthening the flight path, but at the expense of portability.
For the flight time measurement of an instrument to be precise as well as accurate, i.e. such that ions having the same m/e arrive at the detector simultaneously, the kinetic energy imparted by acceleration must be much greater than the statistically random thermal energy of the ions prior to acceleration and the flight region must be shielded from the effects of stray magnetic and nonuniform electric fields which distort the flight path of the ions.
The accuracy of time measurement in a compact instrument can be improved up to a point by improving the time resolution capability of the electronics. Suitable commercial time to digital converters may be utilized in the present invention. Preferably, a highly accurate digital convertor is utilized, such as that disclosed in commonly assigned co-pending application Ser. No. 07/493,507, filed Mar. 14, 1990, hereby incorporated by reference, which improves time resolution by a factor of ten or more over convertors previously employed with TOF instruments. Thus, the flight region of an instrument can, in theory, be reduced by a factor of ten while maintaining the same accuracy.
Though improved time measurement capability has made construction of portable moderate and high resolution mass spectrometers potentially more feasible than previously thought, obstacles to the construction and operation of portable instruments remain.
In their article entitled ".sup.252 Cf-Plasma Desorption Time-Of-Flight Mass Spectrometry," Intern. J. Mass Spectrom. Ion. Phys., Vol. 21, pp. 81-92 (1976), Macfarlane and Torgerson describe a promising combination of a time of flight mass analyzer with a plasma desorption ionizer. In the plasma desorption ionizer, the sample to be mass analyzed is adsorbed onto a surface and bombarded with fission fragments from a radioactive source, .sup.252 Cf. The interaction of fission fragments with the sample ejects and ionizes sample molecules whereupon they are available for acceleration and mass analysis. Unlike more common ionization methods such as electron ionization and chemical ionization, the sample molecules are volatilized and ionized simultaneously.
The .sup.252 Cf nucleus fissions into two fragments which travel in nearly opposite directions. Thus each fission fragment which strikes a sample to induce desorption has a complementary fragment which can be detected and used to generate a start signal for time-of-flight measurement.
Though otherwise ideally suited for ionizing sample in a portable instrument, .sup.252 Cf poses a potential health risk to the instrument operator and Federal regulations restrict its transportation in potentially hazardous quantities. Reducing the size of a .sup.252 Cf source is one way to reduce the safety hazard posed by the instrument. But reducing the size of the fission source likewise diminishes the rate at which fission events occur, which must be compensated by improvements in efficiency. Efficiency can be improved by increasing the probability that a given fission event will produce sample ions. This probability is enhanced when the sample is deposited in a homogeneous thin layer over the entire sample foil. MacFarlane and Torgerson observed that a homogeneous layer of sample could be formed on the sample foil using an electrospray technique. However, to electrospray a sample onto the surface of a sample foil, or other surface, the instrument must be opened to provide access. This requires evacuation of the instrument as each new sample is introduced. In addition, opening the instrument can increase the operator's exposure to harmful amounts of radiation, and reduce the useful lifetime of the detector by exposure to potential chemical contaminants.
It would therefore be highly desirable to deposit a homogenous layer of sample onto a surface where it can be subjected to ionizing radiation without the need for opening the instrument and re-evacuating it with the introduction of each new sample.
Plasma desorption is a gentle ionization method which typically produces only a few types of fragments of which the parent (M+1) ion is present in large proportion, wherein M is the mass of the parent molecule and M+1 occurs due to hydrogen attachment. Accordingly, the plasma desorption method is better adapted to elemental analysis than to molecular structure analysis, for which it needs to be paired to a moderate or high resolution mass analyzer.
Ion resolution is improved in an instrument of a given time resolution by making the flight path of the ions as long as possible. Macfarlane and Torgerson used a plasma desorption spectrometer having a linear geometry. Other linear geometry instruments are also disclosed in U.S. Pat. Nos. 4,490,610 and 4,694,168. In a linear geometry, the flight region is generally cylindrical and bounded at both ends by an ion source and detectors. This geometry requires the instrument to be substantially longer than the flight region. It would be desirable to be able to increase the flight path length in an instrument of a given size by altering the positions of the various components which presently are positioned at the ends of the flight region.